Generally, a tumour is a swelling caused when a cell escapes the controls that regulate growth and division, allowing the production of excess cells. For a cell to become tumorigenic, evidence suggests that it must undergo (a number of) mutations to its genes. Genes that contribute in some way to the tumorigenic transformation of cells are called oncogenes, for example ras, p53 or HER2/neu. Approximately 20% of all human tumours have a point mutation in one of the ras genes (Barbacid (1987) Annual Review of Biochemistry, 56, 779-827) and about 50% have a point or structural mutation to p53 (Harris (1991) Nature 350,377-378). Such gene mutations are reflected in the production of abnormal proteins. It is generally recognized that an immune response might be generated to attach the tumour cell(s), if these abnormal gene products could be identified as such by the immune system.
The immune system has various weapons in its armoury; amongst these are the T cells. T cells recognise complexes of Major Histocompatibility Complex MHC) molecules and processed peptide fragments from internal proteins after these peptides have been carried to the cell surface by the MHC molecules. This enables the T cells to recognise and eliminate cells containing (intracellular) pathogens or other "foreign" antigens. MHC molecules include MHC genes, mRNA, peptides and proteins; however, it is the MHC proteins that present antigen.
In man, MHC genes and products are called HLA genes and products.
In the case of tumour cells, the "foreign" material or antigen can come in a variety of different forms, including the following:
(1) Antigen from a virus causing the tumour (at least in part); PA1 (2) Embryological or developmental antigens expressed by the tumour; PA1 (3) Differentiation molecules or antigens which may be expressed by that particular type of tumour (e.g. melanomas); PA1 (4) Products of oncogenes and/or higher levels of oncogene products in transformed tumour cells. PA1 (a) tumours are often immunosuppressive; PA1 (b) tumour cells often have lost (or at least have reduced levels of MHC molecules; and PA1 (c) levels of antigen may be low.
Theoretically, it should be possible to immunize a patient against any of these four groups of antigens generating a corresponding T cell immune response. Groups 1-3 encompass essentially all of what has previously been covered by the term "tumour antigen" (see Boon et al (1995) Immunology Today, 16,334-336).
If an effective immune response is to be generated, it must avoid simply selecting for tumour cells that no longer express the particular antigen-antigen loss variants of the tumour. This would allow the cancer to return. However, a more effective response may be obtained for the oncogene products (category 4), because the oncogene products are required for the tumorigenic transformation of cells, which means that antigen loss variants either will not occur, or will not be tumorigenic.
Various strategies have been tried to produce an immune response to the products of oncogenes. A (T cell) immune response will have to be specific to the antigen, and so it will be necessary to identify the actual antigen present in the tumour cells. Specific mutations in the oncogenes can be screened for in a variety of ways, for example using mutation-specific monoclonal antibodies, probing with oligonucleotides or utilizing the polymerase chain reaction (PCR), to reveal information about (potential) tumour antigens for each specific patient.
Since most oncoproteins (the products of oncogenes such as ras) are intracellular proteins, the appropriate immune response is a T cell response requiring MHC molecules of the tumour cell to present antigen. Three problems arise in tumour cells:
Immunization must therefore overcome the immunosuppression of the tumour cells, provide suitable MHC antigens, and supply adequate quantities of antigen, in order to produce an effective immune response. Immunological theory is generally that the antigen must be presented to the T cells by "self" MHC molecules, i.e. MHC molecules which are encoded for and produced by the individual itself. Current approaches centre on either immunising with synthetic peptides of the oncogenes or generating HLA-matched peptide specific cytotoxic T lymphocytes (CTL) (Boon et al (1994) Ann Rev Immunol 12, 337-365; Finn (1993) Current Opinion in Immunology 5, 701-704).
Immunization with peptide (WO 92/14756) alone or in adjuvant has been found to provide only limited protection against tumours.
In order to improve immunization, peptides can be pulsed onto autologous antigen presenting cells (APC) and used as a vaccine (WO 94/21287; Crabbe et al (1995) Immunology Today 16, 117-121), but, due to HLA variation, it is difficult to standardise (i.e. to produce a vaccine which will be HLA compatible for random individuals). Further, obtaining fresh APC for each patient is somewhat cumbersome and time-consuming (Yanuck et al (1993) Cancer Res., 53, 3257-3261). Moreover, it is generally believed that APCs cannot be taken from other people, or at least not without rigorous MHC matching, as there is so much variation in MHC genes that every individual has, for practical purposes, a unique combination of HLA alleles.
Alternatively, if T cells can be generated to the products of the oncogenes, they might be used to treat the cancer. Again, it is generally understood that the cells would have to be the patient's own cells, or clearly MHC-matched, and hence this method would be difficult to standardize.
A completely different approach would be to somehow take the patient's own tumour cells and try to use them to immunise against the residual tumour cells. Experimentally, such cells could be made more efficient at presenting the antigen and stimulating an immune response by treating them in various ways, for example transfecting them with various genes (e.g. B7), or immunising together with adjuvants, cytokines or additional strong antigens. The cells would have to be killed first to eliminate the risk of simply injecting tumour cells, thereby causing more tumours.
Since according to immunological theory, an antigen must be presented to the T cells by "self" MHC molecules, there is a strict requirement for self MHC molecules, and tumour cells from other individuals could not be used, or at least not without rigorous MHC matching.
In studies into the treatment of melanoma, melanoma cells from unrelated individuals have been used as a source of melanoma-specific antigen (Morton et al (1996), Tumour Immunology, Ed. Daigleish and Browning, CUP, Cambridge 241-268). Surprisingly, this has had some clinical success, but not without concerns from immunologists and attempts to at least partially match MHC antigens.
In addition, only a very limited array of melanoma-associated antigens (i.e. group 3 above) was chosen, and it is possible that the immune system will not recognise tumour cells that have lost these or are expressing different antigens. This will result in recurring tumours (Kim et al (1992) Int. J. Cancer 51, 283-289). This problem was recognised in WO 95/31107 which discloses a tumour vaccine comprising allogeneic cells expressing cytokines and an array of tumour-associated antigens encoded by a patient's own genomic DNA. The usefulness of this method is limited as many tumour-associated antigens are required and the antigens must be derived from the patient's own tumour cells.
There remains, therefore, a need for a simple vaccine which can elicit a strong immune response for the treatment of tumours.